17

Is it possible to run a Python script as a background service on a webserver? I want to do this for socket communication.

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Robin Rodricks
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6 Answers6

10

You can make it a daemon. There is a PEP for a more complete solution, but I have found that this works well.

import os, sys

def become_daemon(our_home_dir='.', out_log='/dev/null', err_log='/dev/null', pidfile='/var/tmp/daemon.pid'):
    """ Make the current process a daemon.  """

    try:
        # First fork
        try:
            if os.fork() > 0:
                sys.exit(0)
        except OSError, e:
            sys.stderr.write('fork #1 failed" (%d) %s\n' % (e.errno, e.strerror))
            sys.exit(1)

        os.setsid()
        os.chdir(our_home_dir)
        os.umask(0)

        # Second fork
        try:
            pid = os.fork()
            if pid > 0:
                # You must write the pid file here.  After the exit()
                # the pid variable is gone.
                fpid = open(pidfile, 'wb')
                fpid.write(str(pid))
                fpid.close()
                sys.exit(0)
        except OSError, e:
            sys.stderr.write('fork #2 failed" (%d) %s\n' % (e.errno, e.strerror))
            sys.exit(1)

        si = open('/dev/null', 'r')
        so = open(out_log, 'a+', 0)
        se = open(err_log, 'a+', 0)
        os.dup2(si.fileno(), sys.stdin.fileno())
        os.dup2(so.fileno(), sys.stdout.fileno())
        os.dup2(se.fileno(), sys.stderr.fileno())
    except Exception, e:
        sys.stderr.write(str(e))
int3
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Robert
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  • In the code snippet above, I can see at least one problem: sniffing daemon presence isn't done before attempting to launch a fresh instance. – jldupont Sep 14 '09 at 19:54
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    sniffing? Do you mean checking if the daemon is already running? if so, this is done in a different procedure before call the above. – Robert Sep 14 '09 at 21:40
  • nice demostration, very fun ! There's a lot of method to do that python-daemon, subprocess with custom shell calls (nohup, &), but this is very simple to understand. Nice :) – Mychot sad Jan 20 '13 at 16:20
  • in practice, the demonization part may be outside (systemd, supervisord). For a web app, your code is even further removed from the demonization, e.g., all you need is to create a wsgi application without worrying how it is deployed (e.g, [flask deployment](http://flask.pocoo.org/docs/1.0/deploying/)). – jfs May 13 '18 at 08:47
7

You might want to check out Twisted.

Fragsworth
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  • I checked it out, and its quite confusing to grasp. So Twisted is a Python library, which keeps a python script running forever for whatever communication task? (eg. Sockets) – Robin Rodricks Sep 15 '09 at 11:35
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    Twisted is a python library for writing servers. Once you write a server in twisted, you can use 'twistd -y server.py' to run it as a daemon – Nathan Aug 02 '10 at 14:57
2

on XP and later you can use the sc.exe program to use any .exe as service:

>sc create
Creates a service entry in the registry and Service Database.
SYNTAX:
sc create [service name] [binPath= ] <option1> <option2>...
CREATE OPTIONS:
NOTE: The option name includes the equal sign.
 type= <own|share|interact|kernel|filesys|rec>
       (default = own)
 start= <boot|system|auto|demand|disabled>
       (default = demand)
 error= <normal|severe|critical|ignore>
       (default = normal)
 binPath= <BinaryPathName>
 group= <LoadOrderGroup>
 tag= <yes|no>
 depend= <Dependencies(separated by / (forward slash))>
 obj= <AccountName|ObjectName>
       (default = LocalSystem)
 DisplayName= <display name>
 password= <password>

You can start your pythonscript by starting the python interpreter with your script as argument:

python.exe myscript.py
Peter Parker
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2

There is the very helpful Pypi package which is the basis for my daemons written in Python.

jldupont
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0

Assuming this is for Windows, see this recipe based on srvany

mjv
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0

If you are talking about linux, it is as easy as doing something like ./myscript.py &

Unknown
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  • What Andre said. I neglected to mention nohup because I usually just run screen and detach, so there's no need for me to do that. These downvoters are vicious. – Unknown Sep 14 '09 at 20:28
  • But even if you're running screen, you haven't created a true daemon, it's still a child of your screen session. nohup isn't the best solution either, but it's a quick way to run a once off daemon. – JimB Sep 14 '09 at 20:43
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    http://en.wikipedia.org/wiki/Daemon_%28computer_software%29 "a daemon is a computer program that runs in the background". I think this qualifies. – Unknown Sep 14 '09 at 21:18
  • And if you keep reading: "In a Unix environment, the parent process of a daemon is often (but not always) the init process (PID=1). Processes usually become daemons by forking a child process and then having their parent process immediately exit, thus causing init to adopt the child process. This is a somewhat simplified view of the process as other operations are generally performed, such as disassociating the daemon process from any controlling tty. Convenience routines such as daemon(3) exist in some UNIX systems for that purpose" http://en.wikipedia.org/wiki/Daemon_%28computer_software%29 – JimB Sep 15 '09 at 13:11
  • ... ... starting from screen doesn't give the process over to init, and starting from nohup doesn't create the process as a process group leader. http://en.wikipedia.org/wiki/Daemon_%28computer_software%29#Types_of_daemons – JimB Sep 15 '09 at 13:33
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    I'm not saying `nohup ... &` doesn't work, I'm just saying that we should tell people how to make daemons that have the correct parent process, have the correct umask, are in the correct working directory, won't inadvertently grab a controlling tty, and have their output correctly redirected. – JimB Sep 15 '09 at 13:39